1. Field of the Invention
This invention relates to a system for controlling the phase of a signal waveform supplied to the control system of a solid state converter. More specifically, it involves a digital phase shifter which is positioned in a phase-locked loop to generate a phased output signal which is independent of any nonlinearity or amplitude variation of the line reference.
2. Description of the Prior Art
In the control circuitry of a solid state inverter or the like, it is often necessary to sense electrical waveforms in one circuit and then generate a control signal which accurately reflects phase offset from a predetermined reference. A conventional method for sensing the phase error in the waveform in an AC signal is to present the AC signal to a filter which passes only that signal in the frequency range of interest. A phase shifter receives the filtered signal and this signal is combined with a variable threshold signal to generate an output signal whose phase is indicative of the desired phase. However, if the reference signal varies in amplitude or is otherwise nonlinear, the output waveform from the phase shifter tends to be in error.
Of interest is U.S. Pat. No. 3,991,319 issued Nov. 9, 1976 to J. Servos on STANDBY POWER SUPPLY SYSTEM. It discloses a feedback loop for maintaining synchronization between the DC and AC inputs. A phase detector receives a feedback signal from the inverter and also senses the AC line voltage. The output from the phase detector is presented through a phase feedback switch to a voltage controlled oscillator to vary the frequency and phase of the output signal.
Also of interest is U.S. Pat. No. 3,987,359 issued Oct. 19, 1976 to F. Thompson for PHASE ANGLE FUNCTION GENERATOR FOR STATIC VAR CONTROL. This reference describes a system which includes a timing generator for a phase-locked loop including a phase comparator, a filter and a voltage controlled oscillator. Also of interest is U.S. Pat. No. 3,987,360 issued Oct. 19, 1976 to M. Brennen et al on HIGH ACCURACY FAST RECOVERY REFERENCE TIMING SIGNAL FOR A STATIC VAR GENERATOR in which a phase-locked loop is utilized for providing accurate phase information and rapid capture.
Also of interest is the micropower phase-locked loop circuit disclosed in the RCA Solid State-74 Data Book Series on page 226. In this device a phase comparator feeds a low pass filter whose output is presented to a voltage controlled oscillator. An input to the voltage control oscillator is provided to create a frequency offset if desired. The output of the VCO is fed back to the phase comparator to close the feedback loop.
A problem with the heretofore described prior art phase control schemes is that a command or instructional signal which is to change the phase of the output signal is generally combined with the sensed signal at or before the phase comparator. In that this command signal is typically derived from a calibrated reference, amplitude variation in the sensed sinusoidal signal can erroneously appear in the output waveform as a variation in phase. In addition many of the prior art devices will not provide both a leading and lagging phase offset with respect to the sensed signal.